Solvent based coating compositions are known which employ high molecular weight (e.g. 2,000 to 10,000) polymer resins having crosslinking functionality, and a suitable crosslinking agent. Typically, such coating compositions are applied to a substrate, for example, by spraying, and are then cured by baking the coated substrate at an elevated temperature suitable to drive off the organic solvent and to promote the crosslinking reaction. The resulting thermoset coating, if sufficiently humidity and solvent resistant, can provide aesthetic and functional advantages including corrosion protection for the underlying substrate.
Coating compositions comprising such high molecular weight polymer resins typically comprise only 25% to 50% solids content so as to be sprayable or otherwise conveniently applicable to a substrate. The viscosity of coating compositions of higher solids content is typically too high for this purpose. Conventional epoxy ester based automotive vehicle spray primers, for example, typically have a volatile organic content ("VOC") of approximately 540 g/l.
Elimination of the volatile organic solvent portion during curing of these conventional low-solids coating compositions can present toxicity and flammability hazards in some cases. Furthermore, bulk volume of these coating compositions is relatively large and therefore presents undesirable material handling difficulties and added expense. Furthermore, excessive solvent losses and/or solvent recovery equipment add considerable expense to the coating operation. Recently, governmental regulations on hydrocarbon emissions, particularly applicable to automotive coating operations, mandate a significant reduction in volatile organic content for coating compositions. Emissions treatment equipment can be employed to achieve the specified emissions limit. Such treatment presents significant additional expense, however, and there is a great need to provide coating compositions of VOC reduced near to, or preferably even lower than, the levels set by the governmental regulations.
In response to these concerns, high solids coating compositions have been suggested which, typically, employ low molecular weight multi-functional adducts or copolymers in combination with multi-functional crosslinking agents. These high solids coating compositions are less viscous and, therefore, can be applied by spraying, for example, with far lower VOC than was possible with conventional epoxy ester based coating compositions or other conventional coating compositions comprising high molecular weight polymer resins. After application to the substrate, such high solids coating compositions are cured by baking at a cure temperature, that is, at an elevated temperature suitable to drive off the volatile organic content and to promote polymerization and crosslinking of the multi-functional low molecular weight component(s).
In this regard, teaching is found in U.S. Pat. No. 2,568,885 to Dreyfus, which sugguests that highly polymeric materials can be produced by reaction between bifunctional molecules containing as reactive radicals two urethane radicals or two amino, hydroxyl or carboxyl radicals, or one urethane and one amino, hydroxyl or carboxyl radical. Dreyfus employs no crosslinking agent and no crosslinking reaction appears possible with the polymeric compounds formed by Dreyfus.
The teaching in U.S. Pat. No. 4,101,603 to Smith et al suggests high solids coating compositions comprising a polycaprolactone derivative and a mixture of a methylolated melamine (e.g., hexamethoxymethylmelamine) and a low molecular weight polyol and, optionally, solvent and catalyst. The polycaprolactone derivative used in the coating composition of Smith et al can be, among others, the reaction product of polycaprolactone polyol, polyisocyanate and anhydride of a polycarboxylic acid, i.e., a carboxyl modified polycaprolactone urethane adduct which is generally water insoluble.
In U.S. Pat. No. 3,248,371 to Damusis, polyurethane coating compositions are suggested comprising a multi-functional blocked isocyanate-terminated polyether-based urethane intermediate, and as crosslinking/de-blocking agent, hydroxy tertiary amine which can be hydroxy-terminated nitrogen-containing polyether-based polyurethane intermediates.
In U.S. Pat. No. 3,442,974 to Bremmer, a shelf stable epoxy resin system is suggested which comprises a polyepoxide and the adduct of diisocyanate and phenolic hydroxyl compound. It suggests that upon heating this mixture, the phenolic hydroxyl groups are released and react with the epoxy groups forming an ether linkage and a secondary hydroxyl group and the de-blocked diisocyanate reacts with the secondary hydroxyl group. Use in varnishes, for example, is suggested.
Typically, high solids coating compositions yield cured coatings having polymeric networks that differ significantly in structure and morphology from the polymeric networks provided by conventional, low solids coating compositions comprising high molecular weight polymers. The physical properties of the coatings provided by such high solids coating compositions can differ significantly from those of the cured coatings provided by the conventional, low solids coating compositions. In particular, the cured coatings obtained from known high solids coating compositions can be inferior in that they can be less flexible, less solvent resistant, less adherent to the substrate and/or for other reasons provide less corrosion protection for the underlying substrate. Accordingly, it would be highly desirable to provide a coating composition comprising low molecular weight materials suitable for use in high solids, solvent based coating compositions and yet which, upon curing, form coatings having physical properties comparable to those obtained from conventional low solids solvent based coating compositions.
Accordingly, it is an object of the present invention to provide novel coating compositions which cure at elevated temperature, in situ, on the surface of a substrate to form high polymers, that is, polymeric networks similar in structure and morphology to those obtainable through use of conventional low solids, solvent-based coating compositions.
It is another object of the invention to provide a coating composition of sufficiently low VOC to meet governmental guidelines. It is also an object of the invention to provide a coating composition which can be applied to a substrate by spraying or other known method.
It is a further object of the invention to provide a coating composition which has a long shelf-life, that is, a coating composition which is highly stable below cure temperature and which, upon being cured at elevated temperature provides a thermoset coating having excellent humidity and solvent resistance and corrosion protection for the underlying substrate.
It is another object of the invention to provide a method of making a coating on a substrate, which coating has a polymeric network similar in structure and morphology to that provided by conventional low solids solvent-based coating compositions and having similar advantageous physical properties including, for example, humidity and solvent resistance and corrosion protection for the underlying substrate. Additional aspects and advantages of the invention will be apparent from the following description thereof.